remoteproc: k3-r5: support for graceful stop of remote cores

commit 1db4571a37a91dac5c68e6bb3337151028e4d52f from
git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git

Introduce software IPC handshake between the K3-R5 remote proc driver
and the R5 MCU to gracefully stop/reset the remote core.
Upon a stop request, K3-R5 remote proc driver sends a mailbox message,
'RP_MBOX_SHUTDOWN' to the remote R5 core. The remote core is expected
to relinquish all the resources acquired through Device Manager (DM),
disable its interrupts, send a mailbox acknowledgment
(RP_MBOX_SHUDOWN_ACK)to the MPU R5 driver and enter WFI state.
After receiving the ACK from the remote core, R5 driver requests to
reset the remote core through device manager.

Signed-off-by: Hari Nagalla <hnagalla@ti.com>
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>